The present invention relates to a magnetic recording medium and a process for producing the magnetic recording medium, and more particularly, to a magnetic recording medium comprising a maghemite thin film, which exhibits excellent magnetic properties, especially, coercive force squareness S* value and a more enhanced surface smoothness, and a process for producing the magnetic recording medium comprising such a maghemite thin film by a sputtering method.
In recent years, in magnetic recording apparatuses such as hard disk devices, there has been a remarkable tendency that information devices or systems used therefor are miniaturized and required to have a high reliability. With such a recent tendency, in order to deal with a large capacity data, there is an increasing demand for providing magnetic recording media on which information can be stored with a high density.
In order to satisfy such requirements, the magnetic recording media have been strongly required to not only have a large coercive force, but also reduce a distance between a magnetic head and a magnetic recording layer (magnetic spacing).
As magnetic recording media having a large coercive force, there is widely known those comprising a substrate and a magnetic thin film formed on the substrate.
The magnetic thin films which have been already put into practice, are generally classified into magnetic oxide thin films composed of maghemite, etc. (refer to xe2x80x9cTechnical Report of Electronic Telecommunication Institutexe2x80x9d, published by Electronic Telecommunication Institute, (1981) MR81-20, pp. 5 to 12; xe2x80x9cCeramicsxe2x80x9d, published by Japan Institute of Ceramics, (1986) Vol. 24, No. 1, pp. 21 to 24, and Japanese Patent Publication (KOKOKU) Nos. 51-4086(1976) and 5-63925(1993)); and magnetic alloy thin films composed of Co-Cr alloy or the like.
The magnetic oxide thin films are excellent in oxidation resistance or corrosion resistance due to inherent properties of the oxides. Therefore, the magnetic oxide thin films can show an excellent stability independent of change in passage of time, and less change in magnetic properties with passage of time. Further, since oxides exhibit a higher hardness than that of metals, no protective film is required, so that the magnetic spacing of magnetic recording media comprising such a magnetic oxide thin film can be reduced as compared to that of magnetic recording media comprising the magnetic alloy thin film composed of Co-Cr or the like. Therefore, the magnetic oxide thin film is suitable for the production of high-density magnetic recording media.
In view of the above, it has been attempted to enhance a coercive force of the maghemite thin film as one of magnetic oxide films, by using magnetite particles in which cobalt is incorporated therein. However, with the increase in cobalt content, the maghemite thin film tends to be deteriorated in stability independent of change in passage of time due to adverse influences of heat or the like.
Meanwhile, the present inventors have already proposed the invention relating to the maghemite thin film which can exhibits a high coercive force even with a less cobalt content by controlling the spacing of specific plane of maghemite (Japanese Patent Application Laid-Open (KOKAI) Nos. 11-110731(1999) and 11-110732(1999)).
On the other hand, the magnetic alloy thin films have a coercive force as high as not less than about 159 kA/m (2,000 Oe). However, these magnetic alloy thin films are readily oxidized in themselves and, therefore, tend to be deteriorated in stability independent of change in passage of time as well as magnetic properties.
In order to prevent the deterioration of magnetic properties due to the oxidation, a protective film of diamond-like carbon, SiO2 or the like having a thickness of usually about 5 to 10 nm, is formed on the surface of the magnetic alloy thin film, thereby causing the increase in magnetic spacing corresponding to the thickness of the protective film.
In magnetic recording media, in order to reduce the magnetic spacing, it is necessary to reduce the flying height of magnetic head to as low a level as possible, and always cause the magnetic head to be flying stably. In conventional hard disk drive devices, magnetic recording media used therefor have been required to have some surface roughness in order to prevent the magnetic head from being fixed onto the magnetic recording media due to a meniscus force therebetween upon stopping the magnetic head. At the present time, due to the improvement in hard disc systems, magnetic recording media have been no longer required to show such a surface roughness for preventing the magnetic head from being fixed thereonto. However, it has been required that magnetic thin films used in these magnetic recording media have a more excellent surface smoothness.
Also, it is known that the non-smooth surface of magnetic recording media causes media noise. In order to eliminate such a media noise, it is necessary to reduce a surface roughness of the magnetic thin film.
At present, in magnetic recording media using a magnetic oxide thin film, the surface properties of the magnetic thin film largely depend upon those of a substrate because the thickness of the magnetic oxide thin film is as extremely small as not more than 50 nm. Therefore, it has been required not only to use such a substrate having an excellent surface smoothness, but also to develop techniques for further smoothening the surface of magnetic thin film.
Hitherto, as methods of producing maghemite thin films, there are known (1) a method comprising forming a hematite thin film on a substrate, reducing the hematite thin film at a temperature of 230 to 320xc2x0 C. to transform the hematite thin film into a magnetite thin film, and then oxidizing the magnetite thin film at a temperature of 290 to 330xc2x0 C. to transform the magnetite thin film into a maghemite thin film; (2) a method comprising forming a magnetite thin film on a substrate and oxidizing the magnetite then film at a temperature of not less than 320xc2x0 C. to transform the magnetite thin film into a maghemite thin film; (3) a method comprising forming a cobalt-containing maghemite thin film directly on a substrate by decomposing vapors of metal chelate, metal carbonyl or ferrocenes containing iron and MX (wherein M represents at least one element selected from the group consisting of Co, Cu, Rh, Ru, Os, Ti, V and Nb; and X is 0.01 to 0.1) and an oxygen gas in a high-density pressure-reduced plasma by applying a magnetic field thereto (Japanese Patent Application Laid-Open (KOKAI) No. 3-78114 (1991)); or the like.
Presently, it has been strongly demanded to provide magnetic recording media having a maghemite thin film capable of showing excellent magnetic properties, especially coercive force squareness S* and an excellent surface smoothness while maintaining a high coercive force. However, such magnetic recording media satisfying these requirements have not been obtained until now.
Namely, in the above magnetite production methods (1) and (2), the obtained magnetize thin film is taken out into atmosphere, and further subjected to oxidation treatment at a temperature of 280 to 450xc2x0 C. under the atmosphere, thereby obtaining the maghemite thin film. However, since it is necessary to conduct the heat treatment at a temperature as high as not less than 280xc2x0 C., there arises such a problem that the obtained thin film is deteriorated in magnetic properties due to migration from the substrate. Therefore, it is required to select materials of the substrate from those having an excellent heat resistance, so that substrates usable therefor are limited. In addition, since the magnetite thin film is taken out in atmosphere, there also arise problems such as contamination thereof.
On the other hand, in the method (3) of producing the cobalt-containing maghemite thin film, since the maghemite thin film is formed at a substrate temperature as low as about 50xc2x0 C., it is possible to use plastic substrates made of polyesters, polystyrene terephthalate, polyamides or the like having a less heat resistance. However, the obtained magnetic recording media have a very low coercive force, i.e., about 135 kA/m (1,700 Oe) at most.
As a result of the presents inventors"" earnest studies for solving the above problem it has been found that by forming a magnetite thin film on a substrate by sputtering method and then subjecting the magnetite thin film to sputtering treatment in an oxygen-rich atmosphere to transform the magnetite thin film into a maghemite thin film, the obtained magnetic recording medium is excellent in coercive force squareness S* and surface smoothness. The present invention has been attained based on the finding.
An object of the present invention is to provide a magnetic recording medium comprising a maghemite thin film, which is excellent in magnetic properties, especially coercive force squareness S*, and is more enhanced in surface smoothness.
Another object of the present invention is to provide an industrially and economically advantageous process for producing a magnetic recording medium comprising a maghemite thin film by sputtering method.
To accomplish the aims, in a first aspect of the present invention, there is provided a magnetic recording medium, comprising:
a substrate; and
a maghemite thin film formed on the substrate, having a thickness of 7 to 50 nm, a center-line mean surface roughness (Ra) of 0.1 to 1.0 nm and a coercive force squareness S* value of not less than 0.50.
In a second aspect of the present invention, there is provided a magne recording medium, comprising:
a substrate,
an underlayer rmed on the substrate, comprising oxides such as nickel oxide, magnesium oxide or cobalt oxide, chromium metal, or chromium alloy such as Cr-W alloy, Cr-V alloy, Cr-Ti alloy or Cr-Mo alloy; and
a maghemite thin film formed on the substrate, having a thickness of 7 to 50 nm, a center-line mean surface roughness (Ra) of 0.1 to 1.0 nm and a coercive force squareness S* value of not less than 0.50.
In a third aspect of the present invention, there is provided a process for producing the magnetic recording medium according to the first invention, comprising:
forming a magnetite thin film on a substrate by sputtering method; and
continuously subjecting the magnetite thin film to sputtering treatment in can oxygen-rich atmosphere in a sputtering chamber without taking out the thin film into an atmosphere, to transform the magnetite thin film into a maghemite thin film,
In a fourth aspect of the present invention, there is provided a process for producing the magnetic recording medium according to the second invention, comprising:
forming an underlayer on the substrate, comprising oxides having a NaCl-type structure such as nickel oxide, magnesium oxide or cobalt oxide; chromium metal; or chromium alloy such as Cr-W alloy, Cr-V alloy, Cr-Ti alloy or Cr-Mo alloy;
forming a magnetite thin film on the underlayer by sputtering method; and
continuously subjecting the magnetite thin film to sputtering treatment in oxygen-rich atmosphere in a sputtering chamber without taking out the thin film into atmosphere to transform the magnetite thin film into a maghemite thin film.